1. Field of the Invention
The present invention relates to ROM arrays.
2. Description of the Prior Art
ROM arrays composed of columns and rows of memory cells are known in the art. When a data value is required to be read out from such a ROM array, first the row of memory cells containing the memory cell holding the required data value is selected by activating a word line corresponding to that row. Then a bit line associated with the column of memory cells containing the memory cell holding the required data value is used to perform a read operation on the value stored in the selected memory cell. Typically this read operation is carried out by pre-charging the selected bit line and then measuring the discharge that occurs when the relevant memory cell is activated by means of its corresponding word line.
Various techniques have been proposed to achieve high density ROM arrays. In U.S. Pat. No. 5,917,224 a compact ROM matrix is disclosed in which two adjacent columns of memory cells share a virtual ground line. In U.S. Pat. No. 7,002,827 a ROM array is disclosed in which a column of memory cells shares a virtual ground line with an adjacent column of memory cells on one side and shares a bit line with an adjacent column of memory cells on the other side. The densities of memory cells achieved by these two prior art arrangements are not however without associated cost.
For example, in the arrangement of U.S. Pat. No. 5,917,224 the coupling capacitance between the adjacent bit lines can be problematically high, especially for bit lines which have a large number of transistors connected thereto. On the other hand the arrangement of U.S. Pat. No. 7,002,827 requires more complex readout logic, to ensure that the generated data value has been read from the memory cell intended to be read and not another memory cell sharing a bit line or virtual ground line. In both cases the increase in capacitance of the shared virtual ground lines (with respect to non-shared virtual ground lines) can lead to delays in operational speed.
It would be desirable to provide an improved technique for providing high density ROM arrays.